Molecular imaging of living subjects provides the ability to study cellular and molecular processes that have the potential to impact many facets of biomedical research and clinical patient management. Imaging of small animal models is currently possible using positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), computed tomography (CT), optical bioluminescence and fluorescence, high frequency ultrasound, as well as several other emerging modalities. However, no single modality currently meets the needs of high sensitivity, high spatial and temporal resolution, high multiplexing capacity, low cost, and high-throughput.
Fluorescence imaging, in particular, has significant potential for in vivo studies but is limited by several factors. These include a limited number of fluorescent molecular imaging agents available in the near infra-red (NIR) window with large spectral overlap between them which restricts the ability to interrogate multiple targets simultaneously (multiplexing). In addition, background autofluorescence emanating from superficial tissue layers restricts the sensitivity and the depth to which fluorescence imaging can be employed. Moreover, rapid photobleaching of fluorescent molecules limits their useful lifetime and prevents studies of prolonged duration. We have therefore been attempting to develop new strategies that may solve some of the limitations of fluorescence imaging in living subjects.
Raman spectroscopy can differentiate the spectral fingerprint of many molecules, resulting in very high multiplexing capabilities. Narrow spectral features are easily separated from the broadband autofluorescence since Raman is a scattering phenomenon, as opposed to absorption/emission in fluorescence, and Raman active molecules are more photostable compared with fluorophores that are rapidly photobleached. Unfortunately, the precise mechanism for photobleaching is not well understood. However, it has been linked to a transition from the excited singlet state to the excited triplet state. Photobleaching is significantly reduced for single molecules adsorbed onto metal particles due to the rapid quenching of excited electrons by the metal surface, thus preventing excited-state reactions and hence photobleaching. However, the inherently weak magnitude of the Raman effect (approximately one photon is inelastically scattered for every 107 elastically scattered photons) limits the sensitivity, and as a result the biomedical applications of Raman spectroscopy. The discovery of the surface enhanced Raman scattering (SERS) phenomenon offers an exciting opportunity to overcome this lack of sensitivity and introduce Raman spectroscopy into new fields. SERS is a plasmonic effect where molecules adsorbed onto nano-roughened noble metal surfaces experience a dramatic increase in the incident electromagnetic field resulting in high Raman intensities comparable to fluorescence.
Single walled carbon nanotubes (SWNT) also show an intense Raman peak produced by the strong electron-phonon coupling which causes efficient excitation of tangential vibration in the nanotubes quasi one-dimensional structure upon light exposure. Recent demonstration of tumor targeting using radiolabeled SWNT combined with low toxicity effects and rapid renal excretion suggest carbon nanotubes may also become promising molecular imaging agents for living subjects.